1. Field of the Invention
The present invention relates to a process for the preparation of aqueous, radiation-curable urethane-acrylate dispersions, in the absence of solvents; the dispersions prepared by this process; and cured films of these dispersions.
2. Discussion of the Background
Radiation-curable dispersions have been developed in recent years. Their development has resulted from the further development of emulsions initially cured merely by physical means in the direction of systems which are thermally or chemically post-cured. Now, films formed, for example, from unsaturated, aqueous dispersions can be cured and polymerized using UV, laser or electron beam curing in a few seconds or fractions of a second.
Radiation-curable, aqueous polyurethane dispersions as described, for example, in DE-A-3,526,016 have a long shelf life, extending over several years. The cured films have interesting properties when applied to sheet steel and are resistant to solvents.
The preparation of emulsions to meet such demanding requirements is very complex. Initially, saturated and unsaturated polyols are brought into reaction with for example isophorone diisocyanate in order to form prepolymers with NCO terminal groups. The system must be diluted with acetone up to 50% by weight depending on the molecular weight and viscosity. Then follows the chemical incorporation of surface-active, ionic reactants, for example, a 30% strength aqueous sodium salt solution of the appropriate aminocarboxylic acids with the formation of an ionic, NCO-free prepolymer system which is then emulsifiable in water. Finally, the auxiliary solvent, for example acetone, is removed by distillation. Thus, prepolymer formation, dispersion and the removal of acetone by distillation requires 15 to 25 hours, depending on the structure of the reactants.
In addition, it is not possible to reuse the acetone distillate directly, because it contains water.
Against the background of required economy in raw materials and energy, the process described above for the preparation of such dispersions is unsatisfactory. Not only the reaction time and the reaction volume, but also the consumption of solvent and energy are considerable. Moreover, the urethane-acrylate ionomers which are formed are known to be thermally and chemically sensitive, i.e., subject to premature polymerization during prepolymer formation or at least to undergoing gradual changes on standing.
Any simplification of the process is therefore useless if the shelf life of the emulsions, which according to DE-A-3,526,016 is required to be some years, is impaired.
Thus, there remains a need for a process for preparing aqueous, radiation-curable, urethane-acrylate dispersions which is simple, provides savings in time and raw materials, and yields dispersion with good properties and a long shelf life. There is also a need for the dispersions produced by such a process and cured films of these dispersions.